A nuclear reactor generates heat even after a long time being stopped the nuclear reaction inside. This residual heat must be removed by special cooling systems to maintain their integrity. In case this heat cannot be extracted, the temperature rises enough to generate explosive atmospheres of hydrogen, and ends with the core meltdown, and the possible release of radioactive substances into the environment.
In the event of total electric power loss, the only way to maintain the cooling of the core is by means of what is called “natural circulation”. This is a physical process that creates flows in the cooling system pipes by the effect of the temperature difference between the outflow water from reactor, and the returned water from the steam generator tubes. The steam generator is where the heat from the core is exchanged and extracted to the outside as clean steam. This steam can be dissipated into the atmosphere, avoiding a direct communication between the core and the external environment.
Systems designed to mitigate these accidents usually include, among others, an injection system of borated water to the reactor. Their mission is to recover the water level in the reactor to maintain the cooling and, additionally, to maintain the concentration of boron in the water, to ensure an adequate shutdown margin to keep the reactor under the critical level.
This system consists of one or more accumulators containing a certain amount of borated water, which is pressurized with nitrogen at a given pressure. These accumulators are connected to the reactor through an isolation valve (opened by default) and a non-return valve.
Under normal conditions, the pressure in the reactor is higher than the pressure in the accumulator. Under these conditions, the non-return valve remains closed and no injection is performed. However, after a depressurization accident, when the reactor pressure falls under the pressure inside the accumulator, this pressured nitrogen starts to inject the borated water to the reactor, until the complete emptying of borated water. Once emptied, the operator must close the isolation valve and stop this injection.
When the accident occurs with more severe events, as the total loss of electric power, the control of the injection equipment is lost. That means that when the pressure in the reactor continues falling under a certain value, the pressurized nitrogen goes inside the reactor cooling circuit.
This nitrogen inside the cooling system has not any adverse effect on the chemical or radiological activity of the reactor. However, that nitrogen is a non-condensable gas, which finally goes to the higher parts of the circuit, mainly the top of the steam generator tubes. This accumulation of non-condensable gas causes the disruption of the natural recirculation flow, which is the only available way to extract the heat outward. This nitrogen greatly complicates the subsequent cooling of the core and substantially increases the chances of core melting.
To prevent this nitrogen reaches the reactor and the steam generators tubes only two strategies may be taken:
The first strategy is to close the valve connecting the accumulator to the reactor when water injection ends. This strategy has several drawbacks: These valves are normally opened and permanently disconnected to avoid a spurious closure. Its necessary to energize them and give them the closure order. However, during accident scenarios without electric power, its impossible to close. Even if it could be achieved by portable systems, isolation may be done either too soon, so that the borated water injection does not take place completely, either too late, when nitrogen has reached the reactor.
The second strategy is to vent the nitrogen to the atmosphere by means of relief valves. This approach suffers from similar drawbacks as above.
Therefore, its evident the need for some automatic system, which prevents the entry of unwanted nitrogen to the reactor, without requiring any external energy for their operation. Furthermore, the system should automatically recognize the appropriate time for its operation. This allows, on one hand, their unattended operation, and on the other hand, to maximize the effect of the injection of cooling water to the reactor. This also should ensure that nitrogen doesn't go into the reactor system.